In electronic parts represented by semiconductor element and liquid crystal display element or the like, a high purity metal of tungsten (W), molybdenum (Mo), tantalum (Ta), titanium (Ti), zirconium (Zr) and cobalt (Co) or the like and silicide compounds of these metals have been used as a material for constituting electrodes such as a gate electrode or the like and wiring material.
In recent years, those electronic parts have been rapidly advanced. In particular, in technical field of the semiconductor element represented by DRAM (Dynamic Random Access Memory), logic LSI, flash memory or the like, demand for high-integration, high-reliability, highly functional performance, high speed processing has been increased, so that an accuracy or precision in finely working technology required to form the electrodes or wirings has been further emphasized.
Further, in order to meet the above demand, it is essential to reduce a resistance of the material for forming the electrode or the wiring.
Conventionally, as the material for forming the electrodes or the wirings used in LSI, for example, silicide compounds represented by MoSix or WSix or the like have been widely used. However, in these days, there have been eagerly reviewed materials having a lower electrical resistance. Among such the materials, tungsten (W) has a low electrical resistance and is also excellent in heat resistance, so that W has attracted engineer's attention as a future material for constituting the electrodes and wirings.
The electrodes and wirings composed of W can be obtained in such a manner that W thin film is formed on a substrate and then the thin film is worked to form a predetermined wiring pattern by an etching treatment or the like. As a representative film forming method, sputtering method and CVD (chemical vapor deposition) method have been widely adopted.
Conventionally, the sputtering method has been mainly used as the method of forming the electrodes and wirings. In the sputtering method, W sputtering target is subjected to a sputtering operation in a vacuum chamber by utilizing noble gas represented by argon (Ar) or krypton (Kr), whereby the W films are formed.
As for the W film represented by a blanket W, the W film can be also formed by a technology using CVD method. However, the sputtering method has great advantages such that a film forming speed is more rapid, a plasma-damage against a priming film is small, and a handling operation is easy in comparison with CVD method. Therefore, there is a high possibility that the sputtering method will be mainly adopted as a future method of forming the electrodes.
By the way, till the present status, a size of Si wafer used in LSI has been shifted from 6-inches to 8-inches, and now, Si wafer having a size of 8-inches has been mainly used. However, it is estimated that the size of the Si wafer will be further scaled up to 12-inches (diameter of 300 mm) in the near future. Although the size of the sputtering target is different depending on types of sputtering devices, the size of the sputtering target corresponding to 8-inch sized Si wafer is generally 300 mm in diameter. Further, a target size of 400 mm or more in diameter may be required for a wafer of 12-inches class.
As a first problem to be posed by the scale up of the wafer size, an in-plain uniformity in thickness of the thin film formed from a large sized target is lowered. In this connection, “in-plain uniformity” means a uniformity or homogeneity of an entire thin film formed on one plain surface of a wafer having a predetermined diameter. Particularly, in case of the electrode used in LSI, a specific resistance of the electrode is greatly fluctuated depending on a difference in thickness of the film constituting the electrode. As a result, the fluctuation has much effect on characteristics of a transistor. In other words, when the uniformity in thickness of the thin film formed as the electrode is not good, a production yield of LSI is lowered and thus exerting a great damage for the LSI manufacturer.
The in-plain uniformity in thickness of the thin film formed by the sputtering operation is greatly influenced by the sputtering conditions i.e., various parameters such as an input power level, gas pressure, distance between the target and a substrate (wafer) or the like. However, even if these parameters are strictly controlled, the in-plain uniformity in thickness of the thin film, which is attainable by using a conventional sputtering device offered commercially, is limited to about 3%.
As another serious problem, there has been posed a problem that particles (dusts) are liable to generate from the target during the sputtering operation. That is, when the particles generated at the film forming operation or generated after the film formation are mixed into the thin film or remain on the thin film, the following problems arise. Namely, a resistance value of the thin film is changed at a portion where the particle is mixed or remains on the thin film thereby to cause a problem of disconnection or short-circuit when the thin film is assembled as a product. Further, the portion where the particle remains is formed to be a convex shape, and the convex portion is more severely shaved than other portions at a subsequent process such as CMP (chemical mechanical polishing) process or the like, whereby a particle drops off therefrom. As a result, a concave portion having a similar shape of the particle is formed, and a resistance value of the concave portion is also changed thereby to cause the problem of the disconnection or the short-circuit when the thin film is assembled as a product. Furthermore, the concave portion is not properly etched under normal etching conditions in comparison with other normal portions, so that there is posed a problem that an accurate patterning of the circuit cannot be performed.
There are several mechanisms of generating the particles. One case is that an abnormal discharge is occurred at a surface of the sputtering target during the sputtering operation, a molten particle generated by the abnormal discharge is scattered and adhered to the wafer. Another case is that a film re-adhered to an outer peripheral portion of the sputtering target is peeled off therefrom due to heat cycle of the sputtering operation, and the peeled film segments are again adhered to the wafer.
As described above, when the uniformity in thickness of the thin film formed as the electrode is not good or the amount of the generated particles is large, the production yield of LSI is lowered and LSI maker suffers a great damage.
As to also W film, the same problems of the particle generation and the in-plain uniformity in thickness of the thin film formed from the above sputtering target are applied. As the sputtering target for forming the W film, the following W sputtering targets are well known. For example, Japanese Patent Application (Laid-Open) No. HEI-5-93267 discloses a sputtering target having a carbon content of 50 ppm or less, oxygen content of 30 ppm or less, a relative density of 97% or more, wherein crystal grains have a shape collapsed in a predetermined direction.
Japanese Patent Application (Laid-Open) No. HEI-5-222525 discloses a method of manufacturing a sputtering target comprising the steps of: pressing W powder to form a molded body having a relative density of 60% or more, heating the molded body to a temperature of 1400° C. or higher in an atmosphere containing hydrogen gas to form a sintered body having a relative density of 90% or more, and hot-working the sintered body to obtain a relative density of 99% or more.
Japanese Patent Application (Laid-Open) No. HEI-7-76771 discloses a sputtering target having a relative density of 99.5% or more and an average crystal grain size of more than 10 μm up to 200 μm.
However, even if the film forming operation is performed using the above conventional W sputtering targets under predetermined sputtering conditions, an attainable limit of the in-plain uniformity in thickness of the W thin film was about 3% and the particle reduction was not satisfactory indeed.
In recent years, in accordance with an increase of the technical demands for high integration, high processing speed, high reliability required for LSI, it has been essentially required for the material for forming the electrode and wiring to lower the resistance. In view of this demand, the material for forming the electrode has been changed from silicide to a high purity metal. Since the attainable limit of the in-plain uniformity in thickness of the W thin film formed by using the conventionally well-known sputtering target is about 3%, when the size of wafer is further increased, there must be shown a tendency that the in-plain uniformity in thickness of the thin film is greatly deteriorated.
In addition, it is also an important issue to reduce the particles generated from the sputtering target. In particular, as to the size of the particles generated by the abnormal discharge, the particles having a size of 1 μm or more are in the majority, so that the reduction of the particles having a size of 1 μm or more have been strongly demanded in these days.
If these phenomena are not eliminated, the production in the mass-producing line of LSI is greatly lowered and there may be arisen greater loss capability.
The present invention had been achieved to solve the aforementioned problems, and an object of the present invention is to provide a W sputtering target and method of manufacturing the target capable of improving the in-plain uniformity in thickness of the W thin film formed on, for example, a large-sized substrate having a diameter of 8-inches or more.